Certain industrial processes, such as heating a load in a furnace, rely on heat produced by the combustion of fuel and oxidant. The fuel is typically natural gas. The oxidant is typically air, vitiated air, oxygen, or air enriched with oxygen. Combustion of the fuel and oxidant causes NOx to result from the combination of oxygen and nitrogen.
An indurating furnace is a particular type of furnace that is known to produce high levels of NOx. Large quantities of pelletized material, such as pellets of iron ore, are advanced through an indurating process in which they are dried, heated to an elevated temperature, and then cooled. The elevated temperature induces an oxidizing reaction that hardens the material. When cooled, the indurated pellets are better able to withstand subsequent handling in storage and transportation.
The indurating furnace has sequential stations for the drying, heating, and cooling steps. Pelletized material is conveyed into the furnace, through the sequential stations, and outward from the furnace. Air shafts known as downcomers deliver downdrafts of preheated air to the heating stations. Burners at the downdrafts provide heat for the reaction that hardens the pelletized material.
An example of a pelletizing plant 10 with an indurating furnace 20 is shown schematically in FIG. 1. A movable grate 24 conveys loads of pelletized material 26 into the furnace 20, through various processing stations within the furnace 20, and then outward from the furnace 20. The processing stations include drying, heating, and cooling stations. In this particular example, the drying stations include an updraft drying station 30 and a downdraft drying station 32. The heating stations include preheat stations 34 and firing stations 36. First and second cooling stations 38 and 40 are located between the firing stations 36 and the furnace exit 42. Burners 44 are arranged at the preheating and firing stations 34 and 36.
A blower system 50 drives air to circulate through the furnace 20 along the flow paths indicated by the arrows shown in FIG. 1. As the pelletized material 26 advances from the firing stations 36 toward the exit 42, it is cooled by the incoming air at the first and second cooling stations 38 and 40. This causes the incoming air to become heated before it reaches the burners 44. The preheated air at the second cooling station 40 is directed through a duct system 52 to the updraft drying station 30 to begin drying the material 26 entering the furnace 20. The preheated air at the first cooling station 38, which is hotter, is directed to the firing and preheat stations 36 and 34 through a header 54 and downcomers 56 that descend from the header 54. Some of that preheated air, along with products of combustion from the firing stations 36, is circulated through the downdraft drying station 32 before passing through a gas cleaning station 58 and onward to an exhaust stack 60.
As shown for example in FIG. 2, each downcomer 54 defines a vertical passage 61 for directing a downdraft 63 from the header 52 to an adjacent heating station 36. Each burner 44 is arranged to project a flame 65 into a downcomer 54. Specifically, each burner 44 is mounted on a downcomer wall 66 in a position to project the flame 65 in a direction extending across the vertical passage 61 toward the heating station 36 to provide heat for the reaction that hardens the pelletized material 26.
The burner 44 of FIG. 2 is an inspirating burner, which injects fuel and combustion air. The combustion air includes unheated air from the blower assembly 50 and preheated air that is drawn from the downdraft 63 through an inspirator 68. The fuel and combustion air are typically injected at a fuel-rich ratio. This produces high levels of interaction NOx as the unmixed or poorly mixed fuel interacts with the high temperature downdraft air.